As a response to an ancient offence, Athens banned Megarian access to Athenian harbours and marketplaces. The decree was a factor in the outbreak of the Peloponnesian War.
Bouchard, Elsa. "Athens' Megarian Decree". The Literary Encyclopedia. First published 01 March 2010
[http://www.litencyc.com/php/stopics.php?rec=true&UID=11222, accessed 20 June 2013.]